Do you have any idea how to generate a report or how to locate users that sending huge number of emails in my organization?

You can search the log files for that user, read the daily report, run the daily report (it's based on pflogsumm - search the internet for details) to list the specific period that interests you or you could even use one of the may log front ends to see what they throw up or finally you could search the forums (this should always be the first thing you do) for details on how to find a compromised account (if that's what you think you've got). The second thing you should do when posting is to give full details of your currently installed Zimbra version and release and some idea of the hardware and number of users on your system plus any useful information from the log files.